Wood has been a crucial material for construction throughout history. However, due to poor natural durability of wood, it is difficult to use outdoors without any additional treatment. Conventionally, wood has been fully or partially impregnated with preservatives. However, some substances are harmful to mankind and environment, hence, regulated strictly. Therefore, methods for achieving sustainable protection of wood have been required and one method that has been investigated for achieving this has been through chemical modification. 

This doctoral thesis aims to develop a new modification system for solid wood in use class 3. The objective was to develop a wood modification system based on maleic anhydride (MA) and sodium hypophosphite (SHP) that enables exterior use without leaching by weathering. To meet this requirement, the modification should involve formation of stable cross-linking, altering the interaction between moisture and wood, consequently enhancing dimensional stability and biological resistance. 

To test the possibility of using MA and SHP, Scots pine sapwood (Pinus sylvestris L.) was treated with various ratio of chemical reagents, curing temperatures and durations. The treated wood was subjected to repetitive wet-dry cycle to assess its dimensional stability and leachability of chemical reagents. The result indicated formation of a stable cross-linking between wood constituents. 

To further investigate the formation of cross-link, solid-state 13C cross-polarization magic-angle-spinning (CP-MAS) nuclear magnetic resonance (NMR), 31P MAS NMR and X-ray photoelectron spectroscopy (XPS) were employed. The findings indicated that the cross-linking was likely to involve phosphonate (C-P-O) bonds. These results provided a deeper fundamental understanding of the reaction mechanisms between wood, MA and SHP, providing further scope for improved treatment systems in the future.

The impact of the modification on wood-water interactions was analyzed using low-field nuclear magnetic resonance (LFNMR) to study water in the wood at a saturated state. Additionally, the hydrophilicity of cell walls was studied via infrared spectroscopy after deuteration using liquid D2O. The results indicated that the modification reduced the affinity of the wood cell wall to water without altering the number of accessible hydroxyl groups.  

Finally, the modified wood was evaluated for fungal decay resistance, mechanical strength test (bending), and thermal stability. The modification significantly reduced mass loss caused by wood-decaying fungi by limiting the moisture uptake in wood and altering the chemical structure of wood. On the other hand, the modification did not improve resistance to fungal growth on the wood surface, suggesting that nutrient accessibility on surface was not influenced by the modification. A bending test showed that while the modulus of elasticity (MOE) was not affected, modulus of rupture (MOR) decreased to half that of untreated wood. Thermal resistance was improved due to the presence of phosphonate, which can promote the formation of a protective char layer and radical moieties. 

This study demonstrated the potential of modifying wood with MA and SHP to enhance durability, dimensional stability, and fire resistance. The modification formed stable cross-link within the wood components, reducing water interaction and improving resistance to biological degradation. However, the reduction in MOR limits its suitability for load-bearing applications. Despite this, the results suggest that the modified wood could be a viable alternative for non-load bearing exterior applications.

Future research should focus on optimising the modification process by reducing temperature, duration, and solvent use while maintaining performance. Investigating catalysts for the reaction may help address these challenges. Additionally, long-term field testing under real environmental conditions is needed to evaluate the durability and stability of the modified wood. Environmental impact assessments and life cycle analysis will also be crucial for ensuring commercial feasibility and sustainability.